1. Field of the Invention
The present invention relates to static and dynamic silicon device structures which incorporate a very thin membrane or diaphragm and are useful in lithographic and sensor applications. This invention relates in particular to the fabrication of a monolithic silicon membrane device structure in which the membrane and its supporting framework are constructed from a single slab or slug of thick silicon.
2. Description of the Prior Art
U.S. Pat. No. 4,543,266, issued Sept. 24, 1985 to Matsuo et al entitled METHOD OF FABRICATING A MEMBRANE STRUCTURE, describes a structure wherein a thin film which becomes a membrane is formed over one major surface of a substrate by a plasma deposition process utilizing microwave electron cyclotron resonance. The substrate is then removed, other than a portion of the substrate which remains as a frame, so as to form a membrane structure. A dense and high quality membrane is formed at a low temperature and the internal stress of the membrane controlled by varying the conditions under which the plasma deposition process is carried out and by heat treating the thin film after its formation.
More particularly, the membrane mask structure, used for X-ray lithography, comprises a silicon nitride membrane having a thickness of 0.5 to 2 .mu.m and an area of 20.times.20 mm in either or both directions, and a frame portion composed of silicon. The structure is made by a process including the steps of depositing a thin film of silicon nitride onto a silicon wafer, applying a mask pattern to the bottom of the wafer, and etching the patterned wafer in 20% KOH to provide a frame-supported membrane structure.
U.S. Pat. No. 4,622,098, issued Nov. 11, 1986 to Ochiai et al entitled METHOD FOR MANUFACTURING SEMICONDUCTOR STRAIN SENSOR, relates to the manufacture of a semiconductor strain sensor but discloses the steps of forming a blind hole in a silicon substrate by mechanical grinding and wet etching to provide a framed membrane-like structure.
More particularly, a semiconductor strain sensor having a semiconductor substrate having a pair of major surfaces parallel to each other. The substrate is worked to constitute a frame and a thin diaphragm by forming a circular blind hole from one major surface. The bottom surface of the blind hole is formed in a conical shape projecting upward from the edge portion to a central portion thereof. The substrate has a thickness of not less than 0.5 mm and not less than about five times that of the diaphragm. The blind hole is formed by grinding and the inner surface of the blind hole is then etched to eliminate a scratch formed in the inner surface by grinding. Resistance layers are formed on the other major surface of the substrate. Each layer has a piezoresistance which varies in accordance with the pressure applied to the diaphragm.
U.S. Pat. No. 4,701,391, issued Oct. 20, 1987 to Lentfer et al entitled MASK WITH MAGNESIUM DIAPHRAGM FOR X-RAY LITHOGRAPHY, describes a structure wherein a mask for X-ray lithography is formed of a multilayer diaphragm with a patterned absorber layer on the diaphragm. The diaphragm includes a layer of magnesium and at least one intermediate layer.
U.S. Pat. No. 4,680,243, issued July 14, 1987 to Shimkunas et al entitled METHOD FOR PRODUCING A MASK FOR USE IN X-RAY PHOTOLITHOGRAPHY AND RESULTING STRUCTURE, describes a method for manufacturing a mask for use in X-ray photolithographic processes including the step of coating a silicon wafer with a layer of boron nitride. A masking substance is used to coat one side of the boron nitride coated wafer, and the boron nitride is etched off of the other side of the wafer. The wafer is then bonded to a pyrex ring using a field assisted thermal bonding process. During the field assisted thermal bonding process, the silicon is bonded directly to the pyrex. Then a zirconium laYer is used to cover the mask and is selectively etched where it is desired to remove a circular portion of the silicon. Thereafter the silicon is subjected to a semianisotropic etch. The remaining structure includes a pyrex ring bonded to a silicon ring across which a layer of boron nitride is stretched. The layer of boron nitride is subjected to an annealing process and is then coated with an X-ray opaque material.
U.S. Pat. No. 4,668,336, issued May 26, 1987 to Shimkunas entitled PROCESS FOR MAKING A MASK USED IN X-RAY PHOTOLITHOGRAPHY, describes a method of manufacturing a mask for use in X-ray photolithography including the steps of coating a set of wafers with boron nitride. The tension in the boron nitride is measured by using a capacitive probe to measure bowing in a set of test wafers. The remaining wafers are attached to a pyrex ring, and the boron nitride is removed from one side of the wafers. A circular hole is then etched in the wafer, and a layer of tantalum and gold are formed on the remaining boron nitride membrane. The gold is patterned via a sputter etching process. Power is reduced at the end of the sputter etching process slowly to reduce mechanical stress in the mask. The tantalum is then etched via a reactive ion etching process. In this way, an X-ray transparent boron nitride membrane is used to support X-ray opaque gold.
U.S. Pat. No. 4,384,919, issued May 24, 1983 to Casey entitled METHOD OF MAKING X-RAY MASKS, describes a process wherein an X-ray mask is made by forming a thin polymide membrane on a silicon wafer substrate which is then back-etched to form a mask supporting ring of the substrate.
The IBM Technical Disclosure Bulletin, Vol. 20, No. 7, Dec. 1977, pages 2868-2871, contains a publication by Castellani et al entitled "Fabrication of E-Beam Projection and X-Ray Masks On A Support Frame" which describes a process for fabrication of both E-Beam projection masks and X-Ray lithography masks wherein the initial substrate serves not only as a carrier through the fabrication but, upon completion of the fabrication, remains as a rigid supporting ring for the fabricated mask. The active area of the mask is not handled in any way after the mask fabrication is completed so no distortion due to mechanical handling or relaxation of the mask or mask substrate results.
The present invention is distinct from the teachings of the prior art references because it provides a monolithic silicon membrane structure superior to the hybrid structures of the prior art due to its greater structural stability and ease of fabrication.